Minor histocompatibility antigens
Minor histocompatibility antigens are alloantigens with apparently lesser contribution in the allostimulation of recipients by the transplanted organ. They constitute the set of all polymorphic genes (or antigens) different among genetically-different individuals, able to elicit an alloreactive immune response. A classical example of this category of alloantigens is the H-Y antigen system, present in the Y chromosome, therefore only in males. A H-Y mismatch is therefore able to elicit an immune response when female individuals receive organs from male individuals. Although they have lesser immunogenicity than HLA antigens, their contribution is not clinically insignificant as minor histocompatibility mismatches trigger rejection episodes in animal models fully MHC-matched, but at slower pace and strength. Minor histocompatibility antigens are presented to T cells in a similar fashion to conventional environtmental antigens, on the surface of recipient-derived antigen presenting cell, as foreign peptides bound to self-MHC molecules.